An embodiment relates generally to vision-based object detection systems.
Illumination conditions such as shadows can cause errors in the vision-based object detection systems. Shadows distort the color of a captured object resulting in ambiguities between (1) edges due to shadows and (2) edges between different entities (e.g., road and landscape). Applications have been used to filter out shadows but prior art systems assume the use of a camera having a high quality imager. Cameras with high quality imagers are expensive with large packaging sizes, and therefore, not practical especially in a mass-produced vehicle based vision system. With the use of high quality imager, the camera sensor is assumed to be narrow-banded and behave like Dirac delta functions in that they have a non-null response only at a single wavelength. However, a low cost imager typically used in vehicle vision based object detection systems does not conform to the narrow-band sensor assumption. Therefore, previous techniques for shadow removal are inapplicable with the use low cost production imagers.
Traditional approaches for shadow removal such as that described a publication in the International Journal of Computer Vision (IJCV) entitled “Entropy Minimization for Shadow Removal”, describe removing a shadow using invariant imaging. The concept is to select an illumination direction that minimizes entropy. Minimizing entropy does attempt to minimize chaotic disorder of the scattering of the different color variations; however, as entropy is minimized, then all or some of the color values within each color set would attempt to project to a single grouping or cluster. As result, the technique described in IJCV only minimizes variations within a color set or multiple color sets. Maximizing inter-variation between color sets is not a feasible result under this approach described, particularly, when numerous color sets are used. In the IJVC publication, only three distinct color sets are utilized. If more than the three color sets shown are utilized and more specifically, color sets have similar color variations, then the distinction between the color sets would be small which would ultimately prevent the identification of shadow edges within the invariant image.